This invention relates generally to the hermetic packaging of microdevices including semiconductor devices, hybrid devices, vacuum microelectronic devices and Micro Electro Mechanical System (MEMS) devices. The substance of the invention is the use of a high aspect ratio solder perform and an optional mechanical design structure to enhance the surface flow of the solder preform and compression bonding with enough force to create a hermetic cold weld in order to seal a cap and base for the microdevice in a vacuum or a rarefied atmosphere and at room temperature. The cold weld bond is sufficient to ensure adequate hermeticity during the operational lifetime of the device. The resulting yield strength of the cold weld bond is sufficient so as to avoid any other force retention devices.
Hermetic microdevice packages enjoy wide use in the semiconductor industry in applications where true hermeticity is required. Such hermeticity is required where ambient conditions outside of the package or variations in the ambient conditions might affect device performance. In the semiconductor industry, hermetic passivation layers have been developed and applied to the surfaces of sensitive devices in order to give a primary level of defense against ambient conditions. Some of these layers are adequately hermetic. In other instances, a hermetic package is required.
While there exists numerous examples of cold weld feasibility and hermeticity at the macroscopic scale as cited in this disclosure, a quantitative physical model is more elusive due to the complexity of the bond process. Material deformation is a key element for successful cold welding. It is normally characterized by a surface expansion at the interface of each material to be cold welded. N. Bay, Weld Journal, Vol 62, 1983, p. 137, developed a bond strength model based on surface area expansion at the bonding interface and the bonding force normal to the interface. The model relies on the fracturing of a surface interface layer by material shear flow followed by sufficient force normal to the welding interface to force intimate atomic contact between the two members to be bonded. The ratio of bond strength of the stronger member to the yield stress of the weaker member is claimed to be proportional to the extrusion of metal through cracks in the surface layer plus the degree of surface area expansion that will provide the surface layer cracking.
The bond strength model proposed by Bay takes the form of a bond-strength/yield-stress ratio equated to a surface expansion, pressure, yield stress function as follows:
                             Ø            ⁢      b                               Ø            ⁢      o        =                    (                  1          -          β                )            ⁢      Y      ⁢                        p          -                      p            E                                                             Ø                    ⁢          o                      +          β      ⁢                        Y          -                      Y            ′                                    1          -                      Y            ′                              ⁢              p                                           Ø                    ⁢          o                    Øb Bond strengthØo Yield stressβ Area if surfaceY Surface expansionY′ Surface expansion at fracturep Normal pressurepE Extrusion pressureTesting of this relationship has shown satisfactory correlation, (N. Bay, Trans. ASME Jour. Engn. Ind., Vol. 101, 1979), for aluminum to aluminum bonding over the surface expansion range of fractional millimeters.
An important distinction can be made between macroscopic and microscopic shear phenomena. Such distinction can call into question the validity of Bays' relationship for rough surfaces. In cases where there are microscopic asperities in the surfaces to be welded the surface expansion needed to cause welding of the asperities may be very much reduced. Thus there is an issue of how linear the relationship between p-PE and Y-Y′ remains as a function of asperity size and density.
The relationship claimed by Bay identifies some of the key material properties that optimize bond strength and, by extension, vacuum hermeticity. It is clearly advantageous to maximize surface expansion and minimize surface interface layers that can interfere with intimate contact between the two members to be bonded. The bonding pressure normal to the welding surfaces should be maximized within the constraint of the yield strength of the materials to be bonded.
A common hermetic package consists of a package base with electrical feedthrus insulated from such base for the purpose of extracting electrical signals from the device inside the package. The sensitive active device is mounted on the package base and microwire bonds are made to connect the device output pads to the package base electrical feedthrus. Finally a cover or cap is attached to the base via a hermetic bonding technique which varies according to the package material and its preparation. Prior-art bonding techniques require some degree of heat application to insure a hermetic bond of cap to base.
The two most common techniques for hermetically bonding cap to base are cap welding and solder sealing. Cap welding is accomplished by passing a high weld current through a tip (often a small roller) which precesses around the rim of the cap/package assembly as it locally melts the two metal members together. The solder sealing technique utilizes a solder preform (commonly gold/tin eutectic solder) placed between a gold plated cap and base, followed by the application of a heated ring at nominally 320° C. to melt the solder and effect the hermetic seal. Both of these techniques result in a considerable amount of heat transmitted through the package base and to the active device. Although there are methods of reducing the amount of heat transfer to the active device, it is not possible to eliminate the device heating altogether.
MEMS devices which exhibit free-standing micro mechanical structures have been hermetically packaged using both cap welding and solder sealing. However, due to residual stress in free standing members and the extreme sensitivity of structure surfaces, heat can either totally destroy or drastically reduce the production yields of such MEMS devices, especially the more complex types. For such devices, a room temperature package sealing process would be of great benefit.
Room temperature hermetic sealing has been utilized in Ultra High Vacuum (UHV) equipment technology for a number of years and is pervasive in the art of that technology. U.S. Pat. No. 3,208,758, Carlson and Wheeler, describes a vacuum seal technique suitable for high temperature baking after a room temperature seal has been implemented. The patent is focused on large flanges used in UHV vacuum system assembly. A copper gasket seal is described wherein two mating vacuum parts structured with vertical and sloping cutting edges are swaged into the copper gasket to effect a vacuum seal. The high force required for the deformation of the copper is achieved by tightening a series of bolts and nuts around the periphery of a flange. A preferred shape of the cutting edge is disclosed although the force required to effect a vacuum tight seal is not disclosed. The assembly, including the copper gasket and cutting edge shape, has come to be known as a “conflat” type vacuum fitting and is in wide use in the vacuum equipment industry. It has not been applied to microdevice packaging. Additional embodiments of the basic “conflat” sealing technique can be found in U.S. Pat. Nos. 3,217,992, Glasgow, and 3,368,818, Asamaki, et. al., both describing alternative bolting attachment geometries to effect the metal seal. Neither patent addresses the possibility of applying the technique to seal MEMS or microdevice packages.
Macro-scale cold welding has also been used for many years. Cold weld sealing has been exploited for cryogenic applications, such as focal plane array device sealing and input/output signal connections, as is described in A. M. Fowler et. al., “Orion: The Largest Infrared Focal Plane Array in Production”, National Optical Astronomy Observatory, NOAO Preprint Series, No 903. Cold welds may be made with materials having various tensile strengths. While low tensile strength materials require less pressure for the formation of the weld, they are more susceptible to deformation caused, for example, by later elevated temperature excursions. High tensile strength materials require higher pressures for the cold weld, but are less susceptible to later deformations. Once a weld is made with a moderate tensile strength material such as copper, the bond provides a yield strength close to the yield strength of the materials used.
Indium (melting point, MP=156° C.) has been used extensively as a cold weld material in cold temperature electronics, as is described in “NASA Technical Brief, Lewis Research Center Cleveland Ohio, June 1998”. Indium cold weld bumps are used for cold electronics chip input-output connections. The pressures required for forming hermetic bonds are much lower than the yield strength of single crystal silicon. Due to its' low melting point, however, indium is limited in the degree of post bond temperature cycling that can be tolerated. While vacuum hermetic seals have been demonstrated using indium, the resulting yield strength of the sealed parts is low due to the low yield strength of indium (on order of 1000 PSI). Kyle, U.S. Pat. No. 6,413,800 utilizes indium as a cold weld sealant for micropackages but due to the low yield strength of indium he requires an epoxy sealant as an adjunct for a force retention means.
Cold welding of medium tensile strength materials such as copper, aluminum and gold has been employed for considerable time. So-called “butt” cold welding is used to join heavy wire and rod material (see “Dave Nichols, The Welding Institute, TWI website, “Cold Pressure Welding”). This technique applies much higher pressure than for solder systems and relies on substantial flow of the material at the butt ends—that is a lateral flow of up to 2 to 3 times the rod diameter while containing the circumference of the rod. Hand-operated cold welding tools (see for example “Huestis Industrial Corporation, Cold Pressure Welding Tools. 2002”) are designed and used to cold weld nonferrous materials in the 0.08 to 1.20 mm range. While the cold welding of materials such as copper and aluminum is established art in these kinds of macro-scale applications, it has not been utilized in a MEMS or semiconductor micropackage. Furthermore, the pressures required are much higher than the yield strength of silicon, though they may be closer to the yield strength of some ceramics used as MEMS substrates.
With recent advances in MEMS technology leading to more sophisticated devices, efforts are being made to develop suitable packaging technology, both for single MEMS die and packaging at the wafer level. For MEMS devices, packaging at the wafer level is particularly attractive due to the way they are fabricated. Virtually all MEMS devices comprise micro mechanical elements suspended in space. During the fabrication process, these elements must be supported by a sacrificial material, usually through several levels of processing until the end of the fabrication sequence. At the end of the sequence, the sacrificial material is removed, commonly by etching, leaving the micro mechanical members preserved in their design space. This step is known as the “release” process. It is clearly desirable from a cost point of view to perform this release process on a whole wafer rather than individual die. However, once release is performed, the MEMS devices cannot be singulated without the individual mechanical parts being damaged from singulation debris or becoming stuck together (called stiction). The die are also extremely sensitive to contamination during storage and any processing after release but before packaging. Interest has therefore grown in performing release and hermetic packaging at the wafer scale prior to die singulation.
Recent development work in MEMS packaging at the wafer scale has focused on bonding directly to the silicon or other substrate used to fabricate the MEMS. This includes anodic or fusion bonding using heat and high electric fields, eutectic bonding using heat to form a bond between gold or aluminum to silicon, and thermocompression bonding. A novel application of heat has been explored using a polysilicon resistance heater element embedded directly into the MEMS devices.
In U.S. Pat. No. 6,379,988 B1, Petersen and Conley describe a pre-release plastic packaging of MEMS devices wherein the device is encapsulated in a plastic package prior to release. The plastic package can be perforated to allow release in the package using wet or dry etching processes. In a final step, a cover lid is attached to the plastic package by various means common in prior art.
U.S. Pat. No. 6,400,009 B1, Bishop, et. al., discloses a MEMS package and bonding means employing a firewall to form a protective cavity for the MEMS device during heat sealing of top and bottom members of the package. Electrical feedthrus that penetrate the firewall may be made of polysilicon conductive material encapsulated with silicon dioxide. All structures disclosed are fabricated concurrently with the MEMS device. An integral plurality of solder bumps is claimed as a means of strengthening the solder bonded parts. The sealing means described is heated solder sealing.
U.S. Pat. No. 6,627,814B1, David H. Stark, discloses a package with a continuous sidewall with a top surface prepared for solder sealing. A transparent window forms a top cover. The window is prepared with an outer metallic frame suitable for soldering to the base. The solder method requires the application of heat above the melting temperature of the solder. P U.S. Pat. No. 6,639,313 B1, Martin and Harney, discloses a ceramic package with a recess for holding an optical MEMS mirror device. A glass window cover is disclosed which is heat solder-sealed to the ceramic substrate by means of a flexible, folded metal interposer disposed peripherally around the edge of the glass window and ceramic base. Uniquely, the folded metal interposer allows the difference in expansion and contraction between the window and the ceramic to be mitigated during heat cycling. Hermeticity is achieved by heat soldering.
U.S. Pat. No. 6,413,800, Kyle, teaches a cold weld hermetic packaging method in which a metal seal member is disposed on the base of a device package and an organic sealant is placed along the outside of the base outside the metal seal member, or in a variation, on top of the metal seal member. The lid or cap of the package is then pressed onto said sealing structure to form a hermetic seal at room temperature. The metal seal member is made of indium which Kyle teaches is slightly deformed by the pressing of the cap. A preferred organic sealant is epoxy which can later be cured to hardness by the application of UV light. The organic sealant serves two purposes. The first is to apply further lateral pressure to the metal seal as the organic sealant shrinks during the curing process. The second is to hold the seal together during the operational lifetime of the package. eq., force retention. Kyle teaches that neither the metal seal nor the organic sealant are sufficient in themselves to provide a durable hermetic seal. The soft indium metal provides only a weak hermetic seal and is unable to hold the package together without the additional holding force of the organic sealant. The organic sealant by itself would outgas contaminants into the package and allow moisture to leak in and would therefore not provide a true hermetic seal.